The RDW blood test measures how much your red blood cells vary in size, and it is one of the most overlooked lines on a complete blood count. It stands for red cell distribution width, and while hemoglobin or MCV usually get more attention, RDW often changes before other markers do, making it a useful early clue about your red blood cells. In this article you will learn what the RDW blood test measures, how to read a normal, high, or low result, what combining it with MCV reveals, common causes of an abnormal value, and when it is worth a conversation with your doctor.
What the RDW blood test measures
RDW stands for red cell distribution width. It is part of a standard complete blood count covering your red cells, white cells, and platelets and describes how uniform your red blood cells (erythrocytes) are in size and volume. Laboratories generate the result from a histogram, a graph that plots every red cell in your sample by size. When most cells cluster tightly around the same size, the RDW is low. When cell sizes are spread widely across the graph, the RDW is high. If you are new to reading a lab report in general, our guide on how to read your blood test results covers the layout every report shares before you dive into a specific marker like this one.
Doctors use the term anisocytosis to describe a red blood cell population with significant size variation. Picture your red blood cells as a delivery fleet carrying oxygen to every tissue in your body. A uniform fleet works efficiently, while a fleet mixing very small and very large vehicles suggests something in production has changed. The RDW blood test is essentially a quality check on that fleet.
Where RDW comes from: the bone marrow connection
Red blood cells are manufactured in your bone marrow, the soft tissue inside your larger bones, in a continuous process that replaces roughly one percent of your circulating red blood cells every day. Under normal conditions, this assembly line produces cells of a fairly consistent size, batch after batch. Certain nutrient shortages, inflammatory states, or bone marrow disorders can disrupt that consistency, and the resulting mix of cell sizes shows up as a rising RDW well before other changes become obvious. Because RDW reflects this production process directly, it sometimes shifts before hemoglobin levels that carry oxygen through your blood or the red blood cell count fall outside their own reference ranges, which is part of why clinicians value it as an early signal worth tracking over time rather than judging from a single reading.
Normal RDW range and what your report shows
On most lab reports, the RDW blood test result appears as a percentage in the complete blood count section, sometimes alongside a second value called RDW-SD (standard deviation, reported in femtoliters rather than a percentage). The two measure related but slightly different aspects of size variation, and either can appear depending on your lab’s equipment. If your report also lists RDW-CV, that is simply the coefficient of variation version of the same measurement, expressed as a percentage rather than an absolute unit.
For adults, a typical reference range runs from about 11.5% to 14.5%, though this can vary slightly by laboratory and analyzer, much like the ranges you will find across other markers in our broader chart of normal blood test ranges by panel. The table below summarizes how results are usually categorized.
| Result | Typical range | What it generally suggests |
|---|---|---|
| Normal RDW | About 11.5% to 14.5% | Red blood cells are fairly uniform in size |
| High RDW | Above roughly 14.5%, often flagged at 15% or more | Wide variation in red cell size, which can point to a nutrient deficiency or another underlying process |
| Low RDW | Below roughly 11.5% | Red blood cells are unusually uniform; rarely significant on its own |
Always compare your own number to the range printed on your report rather than to a generic chart, since equipment and reference populations differ between labs. A single value only slightly outside the printed range, with no symptoms, is common and often not a sign of disease by itself.
Combining RDW with MCV for a clearer picture
RDW rarely stands alone in clinical interpretation. Reading it alongside the mean corpuscular volume, which measures the average size of your red blood cells, narrows down the likely explanation considerably. This pairing is one of the most useful shortcuts in interpreting a complete blood count, because RDW tells you how varied your cells are while MCV tells you whether the average cell runs small, normal, or large.
| RDW result | MCV result | Pattern often associated with |
|---|---|---|
| High | Normal | Early iron deficiency, before cell size has fully changed |
| High | Low | More advanced iron deficiency anemia, or certain thalassemias |
| High | High | Vitamin B12 or folate deficiency |
| Normal | High | Some cases of chronic liver disease or bone marrow conditions |
| Normal | Low | Anemia of chronic disease, or certain thalassemia traits |
This table describes common patterns rather than diagnoses. The same combination of numbers can have different causes in different people, so a doctor always interprets your complete blood count alongside your symptoms, history, and, when needed, additional tests such as an iron studies panel measuring ferritin and transferrin saturation.
What causes a high RDW blood test result
A result above the reference range is the most common abnormality doctors encounter with this marker, and it is frequently the first clue in a wider anemia workup. Several conditions, from very common to less frequent, can push RDW upward, and most of them trace back to a disruption somewhere in how new red blood cells are built.
Iron deficiency
Low iron stores are the single most frequent cause of an elevated RDW. Iron is essential for making hemoglobin, and when supply runs short, the bone marrow starts producing a mix of red blood cells rather than a uniform batch, which widens the distribution. This is why RDW is often checked alongside a low ferritin result pointing to depleted iron stores.
Vitamin B12 or folate deficiency
Both vitamin B12 and folate (vitamin B9) are required for normal DNA synthesis during red blood cell maturation. A shortage of either nutrient leads to the production of abnormally large cells, called macrocytes, mixed in with normal-sized ones, which raises RDW. Doctors sometimes confirm this pattern with a dedicated folic acid blood test measuring your vitamin B9 status.
Hemolysis and blood loss
When red blood cells are destroyed prematurely, a process called hemolysis, the bone marrow compensates by releasing younger cells known as reticulocytes into circulation faster than usual. These immature cells tend to be larger than mature red blood cells, which widens the size distribution and raises RDW.
Bone marrow disorders and other causes
Myelodysplastic syndromes, a group of bone marrow disorders that disrupt normal blood cell production, can raise RDW along with other abnormalities on a complete blood count. A recent blood transfusion can also temporarily raise the result, since donor red blood cells often differ slightly in size from your own, an effect that usually fades within a few weeks as your own cell population dominates again. Chronic conditions such as inflammatory bowel disease, liver disease, or long-standing kidney disease are additional possibilities a doctor may consider if the cause is not immediately clear. When RDW is elevated together with a larger-than-average cell size, the pattern often overlaps with what is described in our guide to understanding high MCV levels and their common causes, since the two markers frequently move together in vitamin-related anemias.
What a low or normal RDW blood test result means
A normal RDW is a reassuring sign that your red blood cells are fairly consistent in size, though it does not rule out every possible blood condition, since some forms of anemia keep RDW within range even when hemoglobin is low. This is one reason doctors read RDW as part of a pattern across the complete blood count rather than as a stand-alone pass-or-fail number.
A low RDW, meaning your cells are unusually uniform, is uncommon and is not generally considered a marker of disease on its own. It can occasionally appear alongside certain types of anemia, such as anemia of chronic disease or some thalassemia traits, and it is sometimes seen alongside a low MCV result reflecting smaller than usual red blood cells. Even so, the low RDW itself is rarely the finding that drives further investigation. If you see a low result on your report, it is not typically something to worry about in isolation.
RDW blood test high meaning versus low meaning
Because “high” and “low” carry very different weight for this marker, a side-by-side comparison can help you read your own report more confidently.
| Aspect | High RDW | Low RDW |
|---|---|---|
| How common it is | The most frequent abnormal finding | Uncommon |
| Typical clinical weight | Often prompts a look at MCV, ferritin, and vitamin levels | Rarely acted on by itself |
| Frequent associations | Iron, B12, or folate deficiency; hemolysis; bone marrow disorders | Occasionally seen with anemia of chronic disease or thalassemia traits |
| Usual next step | Review alongside MCV and, if needed, iron or vitamin testing | Usually no specific follow-up needed on its own |
When to see a doctor about your RDW result
Most mildly abnormal RDW results are not emergencies and can wait for a routine appointment, but a few situations call for more prompt attention.
- Your RDW is markedly above the reference range rather than just slightly outside it.
- An abnormal RDW appears together with low hemoglobin, low hematocrit, or a flagged MCV on the same complete blood count.
- You have symptoms of anemia such as persistent fatigue, breathlessness on exertion, dizziness, a rapid heartbeat, or noticeably pale skin.
- The result stays elevated across two or more separate tests rather than settling back into range.
- You have unexplained weight loss, ongoing digestive symptoms, or blood in your stool alongside an abnormal result.
If none of these apply and your value sits only slightly outside the printed range with no symptoms, it is reasonable to mention it at your next scheduled visit rather than seeking urgent care. When in doubt, a brief question to your doctor is always the safer choice.
Nutrition and lifestyle for healthy red blood cells
Because nutrient shortages are behind many high RDW results, diet is a practical starting point once a cause has been identified by a doctor. Iron exists in two forms: heme iron, found in red meat, poultry, and fish, which the body absorbs efficiently, and non-heme iron, found in lentils, beans, tofu, and spinach, whose absorption improves when paired with a source of vitamin C. Vitamin B12 comes mainly from animal products such as meat, fish, eggs, and dairy, which is why people following strict plant-based diets sometimes need supplementation. Folate is concentrated in dark leafy greens, legumes, and citrus fruits. Copper, present in nuts, seeds, and seafood, also plays a supporting role in how the body uses iron. Regular, moderate physical activity supports healthy circulation and bone marrow function generally, though it is not a substitute for addressing an underlying nutrient deficiency or medical condition once one has been identified.
Latest scientific advances
Researchers have spent the past few years exploring RDW well beyond anemia, since bone marrow production is sensitive to inflammation and metabolic stress throughout the body, not just to iron and vitamin levels.
One large study looked at how RDW combined with a blood protein called albumin relates to long-term survival in the general population, not just in people already diagnosed with a specific disease. Using health records from more than 460,000 adults across two major national studies in the United States and the United Kingdom, followed for roughly nine to fourteen years, researchers found that people with a higher RDW-to-albumin ratio had a meaningfully greater risk of dying from any cause during the study period, including from heart disease, cancer, and respiratory illness. What this means for you: this does not mean a single RDW value predicts your personal risk of a specific disease, but it supports the idea that RDW reflects broader processes in the body, such as inflammation, alongside its more traditional role in diagnosing anaemia. This is a large, well-designed study, but it describes an association across populations rather than a cause-and-effect relationship for any one person, and researchers note that further work is needed before RDW-based risk scores could be used in everyday practice.
Separately, several smaller studies published in the last few years have examined RDW as a marker of illness severity in specific conditions, including sleep-related breathing problems and certain infections, generally finding that higher RDW values track with more severe disease. What this means for you: these findings reinforce that RDW is a sensitive, general-purpose signal of physiological stress rather than a marker specific to one disease, which is exactly why it is always interpreted together with your symptoms and other blood values rather than read alone. These are earlier-stage findings, still being confirmed across larger and more diverse groups of patients, so they should be seen as an active area of research rather than a settled clinical guideline.
Glossary
| Term | Definition |
|---|---|
| RDW (red cell distribution width) | A measure of how much your red blood cells vary in size, usually reported as a percentage. |
| Anisocytosis | The medical term for a red blood cell population showing significant variation in size. |
| MCV (mean corpuscular volume) | The average size of your red blood cells, usually read together with RDW. |
| Complete blood count (CBC) | A common blood test that counts and describes red blood cells, white blood cells, and platelets. |
| Haemoglobin | The iron-rich protein inside red blood cells that carries oxygen around the body. |
| Reticulocyte | A young, immature red blood cell recently released from the bone marrow, typically larger than a mature cell. |
| Macrocyte | A red blood cell that is larger than normal, often linked to vitamin B12 or folate deficiency. |
| Thalassaemia | An inherited condition affecting hemoglobin production that can cause anemia of varying severity. |
| Myelodysplastic syndrome | A group of bone marrow disorders that disrupt the normal production of blood cells. |
| Reference range | The span of values considered typical for a healthy population, which can vary slightly between laboratories. |
Frequently asked questions
What level of RDW is considered dangerous?
There is no single RDW number that automatically signals danger. Most laboratories flag results above roughly 14.5% to 15% as high, but the significance of any single value depends on how far it sits outside your lab’s own range, whether other markers such as hemoglobin or MCV are also abnormal, and whether you have symptoms. A markedly elevated result, especially one that persists across repeat testing or comes with symptoms like fatigue or breathlessness, is more worth discussing promptly than a value just above the upper limit. Only a doctor reviewing your full report can say whether a specific number needs urgent attention.
Can medications affect an RDW blood test result?
Yes, certain medications can influence RDW. Some chemotherapy drugs that affect bone marrow function can raise it, and some long-term antiviral medications have been associated with changes as well. On the other hand, successful treatment of an underlying deficiency, such as iron therapy for iron deficiency anemia, typically causes an elevated RDW to gradually decrease as new, more uniform red blood cells are produced. It is worth telling your doctor about every medication and supplement you take when discussing an abnormal result, since this context helps with interpretation.
Is a high RDW ever a sign of cancer?
RDW is not a specific marker for cancer, and an elevated result far more often reflects a nutrient deficiency, inflammation, or another benign process. That said, some research has noted a statistical association between a persistently high RDW and certain cancers, likely related to chronic inflammation or the nutritional effects of illness rather than a direct cause-and-effect link. A single high RDW result is not a cancer diagnosis and should not be treated as one, but a value that stays elevated without an obvious explanation, especially alongside other concerning symptoms, may prompt your doctor to investigate further.
Does RDW change with age or between men and women?
RDW can rise slightly with age, and older adults sometimes show marginally higher values without this reflecting disease. In women of childbearing age, RDW can fluctuate somewhat in relation to menstrual blood loss and iron stores, though it should generally stay within a reasonable range. These are typically small, gradual shifts rather than dramatic changes, so a markedly abnormal result still warrants the same attention regardless of your age or sex.
How can I lower a high RDW naturally?
The most effective approach is to treat the underlying cause once your doctor has identified it, since RDW itself is not something to manage directly. If iron, vitamin B12, or folate deficiency is confirmed, correcting that deficiency through diet, supplements, or medical treatment usually brings the RDW back toward the normal range over the following weeks to months as newly produced red blood cells become more uniform. Trying to lower RDW through diet alone, without knowing the cause, is not recommended, since the right nutritional approach depends entirely on which deficiency, if any, is driving the result.
Can RDW increase during pregnancy?
Yes, a mild increase in RDW is common during pregnancy, particularly in the second and third trimesters, related to expanding blood volume and higher demands for iron and folate. This is generally considered a normal physiological adjustment rather than a cause for concern. A more pronounced increase, however, may warrant closer monitoring to check for and address a true nutrient deficiency, so pregnant patients with an elevated result should still discuss it with their prenatal care provider.
Further reading
- CBC versus CMP and how the two panels differ
- high vitamin B12 levels and what causes them
- low MCHC results and their common causes
- a high total iron-binding capacity result
- what a full blood panel checks beyond the CBC
Sources
- RDW (Red Cell Distribution Width) — MedlinePlus, National Library of Medicine: https://medlineplus.gov/lab-tests/rdw-red-cell-distribution-width/
- RDW Blood Test: What It Is, Procedure and Results — Cleveland Clinic: https://my.clevelandclinic.org/health/diagnostics/22980-rdw-blood-test
- Complete Blood Count (CBC) — Mayo Clinic: https://www.mayoclinic.org/tests-procedures/complete-blood-count/about/pac-20384919
- Hao M, Jiang S, Tang J, et al. — Ratio of Red Blood Cell Distribution Width to Albumin Level and Risk of Mortality — JAMA Network Open, 2024: https://doi.org/10.1001/jamanetworkopen.2024.13213
A complete blood count like the one that reports your RDW often includes several related values worth understanding together, such as hemoglobin, MCV, and iron markers like ferritin. Reading these numbers side by side, rather than in isolation, helps you understand, not diagnose, what might be happening with your red blood cells, and it can help you ask better questions at your next appointment. This kind of interpretation helps understand your results, but it does not diagnose you and does not replace your doctor’s judgment.
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